The present invention relates to a soldering tool particularly adapted to effectively solder, in a single swift operation, connecting tags of an integrated circuit microwafer, i.e., a "chip", to corresponding ends, or terminals, of conductors carried by a substrate, such as a printed circuit board or the like.
In the prior art, the connecting tags have been soldered by a technique similar to that employed for the microwafer contacts. Frequently, ends of the connecting tags have been coated with a suitable soldering or brazing material. If the ends of the tags were not coated, at least the terminals of the conductors on the substrate have been coated with the brazing or soldering material, as have all of the conductors on the surface of the substrate. Also, the geometrical positions of the terminals correspond to the geometrical positions of the connecting tags on the microwafer.
In the soldering process, once the chip is positioned above the substrate, either in contact with it or at a very short distance from it, with the ends of the connecting tags situated above the terminals of the conductors, the soldering tool is lowered a "bit" on the tool presses the ends of the connecting tags against the terminals. Current is then applied to the bit and Joule effect heating of the bit causes the soldering material on the tags to melt, whereby soldering takes place. The tool is then raised back to its initial position. This process is preferably automated, in the prior art and in the present invention.
For the method to be effective, the soldering tool needs to have, in addition to satisfactory mechanical rigidity as far as soldering "bit" is concerned, the lowest possible thermal inertia and complete heating uniformity of the "bit" surface which bears against the connecting tags and terminals. The reason for this is to ensure that the localized melting of the soldering or brazing material is effective and sufficiently fast to prevent re-melting the previously soldered region attached to the other ends of the connecting tags to the terminals on the chip (even if the melting point of the first material is higher than that of the second material).
The object of the invention is to provide a micro-soldering tool which completely satisfies the above requirements and to a method of making same.
A prior art micro-soldering tool which operates by Joule effect comprises a pair of conductive metal or alloy uprights which sandwich an insulating strip between them. These uprights are respectively connected to the poles of a current generator. The sandwich forms the shank of a soldering bit with which it is mechanically and electrically connected. The soldering bit is generally rectangular and made of a material selected from the group consisting of tantalum, titanium, molybdenum and tungsten or their alloys. In one embodiment, described in French patent application Ser. No. 75.10132, filed on April 1, 1975 by Compagnie Honeywell Bull for a "Soldering Tool Operating by Joule Effect," the rectangular bit comprises a small plate, having a roughened surface; the plate is connected to the two uprights of the current supply sandwich by lateral side pieces which, together with the bit, are machined from a sheet of a material selected from the above mentioned group. The sheet is cut and then folded so that the side pieces which are attached to the outer faces of the uprights support the soldering bit at a suitable distance from the lower face of the sandwich. Advantageously, the bit is then machined to define the surface which is applied to the chip. The machining assists in enabling heat to be substantially uniformly applied to the chip by creating higher electrical resistance "border" around the bit, which border thus heats up more rapidly. To increase the speed of heating and cooling (when the current is switched off) the side pieces may be cut in the shape of a trapezoid having a minor base bordering on the bit.